Atypical scrapie – is it more difficult to find than we thought?

February 18, 2011

A few weeks ago I blogged about one type of prion disease: vCJD so I was in two minds as to whether to blog about a second prion disease so soon but having gone through this paper titled ‘Atypical/Nor98 scrapie infectivity in sheep peripheral tissues’ by Andreoletti et al., I thought their results were too interesting and too important to ignore.

Scrapie is a fatal TSE (Transmissible Spongiform Encephalopathy) found in sheep. The ‘classical’ form was the first to be discovered. Research showed that sheep with certain alleles encoding the PrP (prion protein ) were particularly susceptible and sheep with certain other alleles were particularly resistant to infection. This led to the selective breeding of sheep flocks to increase their resistance to scrapie.

In 1998 a second type of scrapie was discovered, ‘atypical scrapie’. Unfortunately the sheep most resistant to classical scrapie are apparently highly susceptible to atypical scrapie.

In my blog post about vCJD I discussed briefly how it originated from the consumption of BSE-infected cattle. This finding led to the exclusion of tissues where the abnormal protein was found in high levels including brain and spinal cord from the food chain for all ruminants.
It is important to note that neither form of scrapie has ever been shown to affect humans.

On to the paper…

Previous research has suggested that non-CNS tissues, including lymphoid tissues and skeletal muscle contain much lower levels of abnormal prion than CNS tissues and in the case of atypical scrapie no detectable abnormal PrP has been found in these tissues. This study investigated further the possibility that these peripheral tissues could be infectious and compared the data they got from atypical cases with classical scrapie cases.

What they did

1) They took samples from 7 naturally occurring atypical scrapie cases.
2) They prepared homogenates of brain tissue from each of these 7 cases and inoculated them into the brains of transgenic mice (carrying the sheep allele for PrP found to be most susceptible for classical scrapie). This is basically a test of infectivity: can the abnormal prions inoculated generate further abnormal prions to the level at which disease is observed.
3) They repeated 2 using lymphoid tissue in the place of brain tissue.
4) They repeated 2 and 3 using 2 experimentally infected sheep.
5) They repeated 2 and 3 using cases of classical scrapie.

What they found

1) In each of 7 natural atypical scrapie cases they could detect accumulation of PrPSc (the abnormal prion) in several brain areas but they couldn’t detect any abnormal prion protein in the lymph nodes they tested.
2) The brain homogenates inoculated into the mice were all found to be positive for TSE (which is reassuring as they were all positive on the test in 1!).
3) 5 out of 7 of the lymphoid homogenates were found to be positive for TSE (which is surprising because all 7 were negative on the test in 1). However the clinical onset of disease in these mice was delayed.
The brains from the mice of BOTH groups displayed similar PrPSc results when tested using the test in 1 and when looked at under the microscope.
4) Both experimentally infected cases were again testing as positive on the test used in 1 for CNS tissue but not peripheral tissue. In the mice assay some of the lymphoid tissue (but not all), striated muscle and peripheral nerves from both cases were positive for TSE.
5) In agreement with previous research, both CNS and peripheral tissues (including lymph nodes and muscle) from the sheep infected with classical scrapie DID test as positive for TSE. When inoculated into mice the peripheral tissue homogenates resulted in prolonged incubation periods compared to the CNS samples.

What they did next

The researchers wanted to know why they couldn’t detect PrPSc in peripheral tissues in atypical scrapie cases even though these tissues proved to be infective in mice so they prepared dilutions of sheep brain homogenates from both aypical and classical scrapie cases and tested them using an OIE registered test and a second test that is commercially available. They found that they could dilute the brain homogenates from classical scrapie cases much more and still detect them than they could the atypical case homogenates (1 in 1000 compared to 1 in 80).

What they concluded
 The diagnostic assays currently used to detect scrapie cases are much less sensitive for atypical scrapie than classical scrapie.
 This may mean that the actual prevalence of atypical scrapie is greater in the sheep population than has been previously thought.
 Previous assumptions that atypical scrapie is poorly/not transmissible may be wrong. (These were based on comparing prevalence in the general population to prevalence in flocks where an atypical scrapie case was diagnosed. When tested, the prevalence in the flock with the infected animal has not been significantly different to the prevalence in the general sheep population, but if the test is not very sensitive for atypical scrapie, other cases in the infected flock could be being missed.)

Future research

The researchers report that we need to know more about the transmissibility of atypical scrapie and suggest looking at its transmissibility by the oral route and looking for the presence of PrPSc in atypical cases in the placenta, colostrum and milk.

What it all means

This is important research because it highlights that peripheral tissues, including the skeletal muscle we eat, show scrapie infectivity in both classical and atypical scrapie cases. As I mentioned earlier, there is no evidence that either form of scrapie can cross the species barrier into humans, but work done using mice over-expressing the pig PrP showed that they could be infected by atypical scrapie so we cannot yet rule out the disease crossing the species barrier into other animals.

As the authors say, “[these results] underline the urgent need for further investigations on the potential capacity of [atypical] scrapie to propagate in species other than small ruminants”.